ePTFE sheets obtained by laminating expanded porous polytetrafluoroethylene films to a prescribed thickness have excellent characteristics provided by polytetrafluoroethylene materials (hereinafter referred to as PTFE), such as chemical resistance, heat resistance, purity, and smoothness. In addition, as a result of making PTFE porous by expansion, the ePTFE sheets have excellent toughness, cushioning properties, and heat insulation properties. Therefore, the ePTFE sheets have been used in various fields including cushioning materials, heat insulation materials, slide materials, and various cover materials. In particular, gaskets are one of the representative applications of the ePTFE sheets.
It has been known that gaskets (hereinafter referred to as ePTFE sheet gaskets), which are cut out from a sheet obtained by layering expanded porous polytetrafluoroethylene (ePTFE) films to a prescribed thickness and integrating the layered ePTFE films by thermal compression bonding, has improved creep resistance and also has excellent fitting properties, when compared to skived PTFE gaskets obtained by cutting out from a compression-molded product obtained from a PTFE molding powder (e.g., Japanese Utility Model Laid-Open Publication No. 03-89133 and Japanese Patent Laid-Open Publication No. 10-237203). ePTFE films are obtained by molding a mixture of a PTFE fine powder and a lubricant, removing the lubricant, expanding an obtained product at a high temperature, and if necessary, baking an obtained product (Japanese Patent Laid-Open Publication No. 46-7284). As shown in FIG. 1, biaxially-expanded ePTFE films are formed of nodes (connections) 2, which are scattered about in an island-like pattern, and fibrils 1, which connect these nodes 2 and which are extended all around in a mesh pattern. The nodes 2 are identified as folded crystals of PTFE. It is thought that the fibrils 1 are linear molecule chains resulting from this fold being unraveled and pulled out by expansion. The above-described sheets obtained by laminating ePTFE films (hereinafter referred to as ePTFE film-laminated sheets) have improved creep resistance when compared to skived PTFE sheets; however, a further improvement in their creep resistance has been desired.
In Japanese Patent Laid-Open Publication No. 11-80705 and Japanese Patent Laid-Open Publication No. 2006-524283, it has been found that creeping is caused by nodes, and it has been proposed that creep resistance can be improved by making the nodes as small as possible. Thus, ePTFE sheet gaskets in which a large amount of fibrils were pulled out by increasing expansion ratios have been developed in Japanese Patent Laid-Open Publication No. 11-80705 and Japanese Patent Laid-Open Publication No. 2006-524283. As shown in FIG. 2, in these ePTFE sheet gaskets, nodes 3 connected by fibrils 1 are made extremely small and have a structure which cannot be called “nodes” any more (hereinafter, such a structure is referred to sometimes as a node-less structure).